1P19.] 21 



remains quite still ; if it is touched, the caterpillar moves a little, but 

 soon becomes immobile a^ain. This enforced immobility is caused 

 possibly by the movements of the larvae inside the caterpillar's body ; 

 at some signal all the larvae within the latter become active pre})aratory 

 to their exit. 



The reason the unfortunate host does not struggle at this stage is 

 possibly due either to a partial nervous paralysis or, more likely, to a 

 rapture of many of its body-wall muscles. That the latter explanation 

 is near the truth is made more likely by the facts that in many cases 

 the exit of the parasites is preceded, first, by violent sti'uggles of the 

 host, and then b}' a period of immobilit}^ and by the additional fact that 

 the head and prothoracic regions of the caterpillar throughout the periods 

 of boring outward, make a slight but constant movement ; this region 

 of the caterpillar is rarely punctured by the out-boring parasites 

 (text-fig. 1). 



After the caterpillar is immobile, examination of its body-wall at a 

 favourable time may show a large number of round pale whitish spots. 

 These pale areas are caused by the contained parasites beginning to eat 

 or gnaw their way outwards ; in doing this they firstly gnaw away the 

 muscles and hypoderm. As the latter is pigmented (yellow or brownish 

 black) the scraping away of the cells causes the round areas to appear. 

 In text-fig. 2 the mesothorax above the letter j:; is seen to have in the 

 mid-lateral region a round pale area, caused as explained above. Watched 

 under a binocular microscope the somewhat pigmented head of the para- 

 site may be observed underneath moving to and fro as it rasps at the 

 cuticle of its host. After the first appearance of these round areas 

 the parasites take about fifteen minutes to penetrate through the 

 remaining outer chitinous cuticle. 



In seveml cases I have been enabled to examine the stages in the 

 exit of these Braconid larvae from the body of the Pieris hrassicae 

 caterpillar, and I have carefully noted the times at which the parasites 

 were at special stages in their process of exit. In the following para- 

 graphs I give a specimen of these observations, one of which began at 

 9.30 A.M. The exit of the parasites may take place at any time of the 

 night or day. 



1. The outward migration of the parasites began at 9.30 a.m. 



2. From the time of appearance of the white scraped areas to the com- 



plete peuetration of the body-wall, 30 niiuutes passed. 10 a.m. 



3. From the time of first breaking through of the head of the parasite to 



the extrusion of half its body, 30 niiiuites passed. 10.30 a.m. 



